The equation of a circle $C$ is $x^2+y^2+2x+8y+13 = 0$. What is its center $(h, k)$ and its radius $r$ ?
Explanation: To find the equation in standard form, complete the square. $(x^2+2x) + (y^2+8y) = -13$ $(x^2+2x+1) + (y^2+8y+16) = -13 + 1 + 16$ $(x+1)^{2} + (y+4)^{2} = 4 = 2^2$ Thus, $(h, k) = (-1, -4)$ and $r = 2$.